During motor speed control, a control gain must be increased in order to improve a speed response and a robustness with which speed variation accompanying load variation is suppressed. However, a motor speed signal obtained from an encoder or a resolver includes noise, and therefore, when the control gain is increased, the noise is amplified, leading to a reduction in the stability of the motor speed. Hence, there is a limit to the control performance that can be achieved simply by increasing the control gain.
To avoid this situation, means for reducing the noise included in the motor speed signal by inserting a low pass filter (an LPF) is typically employed. However, when a cutoff frequency of the LPF is reduced in order to improve the noise reduction effect, a phase of the motor speed signal is retarded such that the speed response deteriorates.
To improve the control performance, therefore, appropriate motor control constants to be set in a motor control apparatus must be determined in consideration of the tradeoff between the control gain and the cutoff frequency of the LPF.
Here, a method of determining motor control constants through automatic calculation simply by applying a single parameter defining a desired response speed, thereby ensuring that the motor control constants are not determined by trial and error due to the tradeoff described above, has been proposed in the prior art (see PTL 1, for example).
More specifically, in the conventional technique described in PTL 1, the motor control constants are determined automatically by applying a target response frequency ωf as the parameter defining the desired response speed. Note that the motor control constants serving as the subject of this conventional technique include a position loop gain of a position control unit of the motor control apparatus, a speed loop gain and a speed integration time constant of a speed control unit, a filter constant of a torque filter unit, a current loop gain and a current integration time constant of a current control unit, and a filter time constant of a speed signal creation unit (in other words, an LPF).
Further, in PTL 1, a speed control loop is considered as a secondary system represented only by the speed loop gain and a motor load inertia, and the speed loop gain is determined so that a characteristic equation of a transfer function having a range that extends from a target speed (a speed command) to the motor speed (an actual speed) has a repeated root. Furthermore, a calculation expression for determining the filter time constant is defined by trial and error on the basis of a stability condition of a control system and a repeatedly performed experiment.